1. Field of the Invention
The present invention relates to Schottky barrier semiconductor devices having a high reverse breakdown voltage.
2. Description of Prior Art
The Schottky barrier diode (hereinafter briefly, SBD) has been widely known, comprising a metal and a semiconductor, wherein an electrostatic barrier is formed at the interface which causes the metal-semiconductor interface to have rectifying properties. SBD's have good high frequency characteristic and are operable with a small forward voltage drop. The recent development in metal-semiconductor contact techniques and semiconductor surface treatment techniques has enhanced efficiency in the production of SBD's, with the result that an increasing number of SBD's have come to be used for signal processing in a high frequency region, as well as for rectifying in power supply devices.
In practical applications, however, the use of SBD's is considerably limited because their reverse breakdown voltage is not high enough. As generally accepted, the low reverse breakdown voltage is referred chiefly to the structure of the SBD. A typical SBD is such that a surface protecting film made of an insulating material such as silicon oxide is formed on one main surface of a semiconductor substrate, and a barrier metal is brought into contact with the semiconductor substrate via an opening formed on the surface protecting film, thus forming a Schottky barrier in the metal-semiconductor interface. In such structure, as shown in FIG. 1, an electric field is readily concentrated in the periphery A in the interface between the barrier metal 1 and the semiconductor substrate 2, causing the reverse breakdown voltage to be lowered.
Several prior art approaches to this problem have been known, including the structure in which, as shown in FIG. 2(a), a p.sup.+ guard ring 4 is formed in the interface A between the barrier metal and the semiconductor substrate where an electric field is readily concentrated, and the structure in which, as shown in FIG. 2(b), a double-guard ring is formed by forming an n.sup.+ guard ring inside the p.sup.+ guard ring. Another prior art approach resorts to modifying the planar structure, one most practical example of which is shown in FIG. 2(c) in which the surface of a semiconductor substrate such as n-type silicon wafer is etched shallowly, i.e., to a depth of 2000 to 3000 A, whereby charges and strain in the boundary between the semiconductor surface and the insulating film formed on the substrate are removed and thus lowering of the reverse breakdown voltage due to such boundary charges and strain is prevented. Another prior art approach is shown in FIG. 2(d) in which a hollow 6 is formed by etching to a depth rj (1 to 10 .mu.m) on part of the main surface of a semiconductor substrate 2 such as n-type silicon wafer to hamper an electric field from being concentrated in the periphery of the interface between the barrier metal and the semiconductor and thereby to improve the reverse breakdown voltage.
In practice, however, the foregoing prior art structures are not very practicable for the following reasons. In the guard ring structure, although the reverse breakdown characteristic can be improved, the high frequency characteristic is impaired by the injection of minority carrier because a p.sup.+ n junction is formed between the p.sup.+ guard ring 4 and the n-type silicon wafer 2. In the double guard ring structure, although the reverse breakdown characteristic can be improved, no substantial improvement in the high frequency characteristic is available because P.sup.+ n and n.sup.+ p.sup.+ n junctions are formed between the guard ring and the semiconductor substrate. Furthermore, in the guard ring structure, the semiconductor substrate must be thicker as deeper the gurard ring is formed in the n-type silicon wafer substrate, and the distribution of impurity concentration in the substrate varies due to thermal diffusion process necessary for forming the guard ring, resulting in a large forward voltage drop. Further, the thermal diffusion process adds an extra work to the forming of the semiconductor device and leads to a rise in the production cost. In the structure in which the main surface of the semiconductor substrate is shallowly etched to preclude influences of boundary charges and strain upon improvement in the reverse breakdown characteristic, it is practically impossible to obtain substantial improvement in the reverse breakdown characteristic. Shallowly etching the semiconductor surface is not an efficient way of improving the reverse breakdown characteristic, or does not give a substantial solution to the problem ascribed to conditions under which the surface protecting film is formed. After all, the advantage available with this technique is not very significant or nothing better than that available with the known planar structure.
In the structure in which the surface of the semiconductor substrate is etched deep, the concentration of an electric field in the periphery of the interface between the barrier metal and the semiconductor substrate can be effectively reduced to enable the reverse breakdown voltage to be raised to a theoretically obtainable value. In experiments, this effect has been attested on PN junction planar devices.
In such structure, however, a flange-like portion 7 over the edge of the hollow remains as part of the surface protecting film 3 as shown in FIG. 2(e) after forming of the hollow by selectively etching the semiconductor substrate, with the result that the barrier metal 1 can hardly be bonded to the entire wall of the hollow and a void 8 where the barrier metal 1 is absent tends to be set up underneath the flange-like portion. This has made it difficult to realize substantial reverse breakdown characteristic better than one obtainable with the known planar devices. One prior art solution to this problem has been to resort to a method in which a barrier metal is bonded to the wall of the hollow by sputtering techniques called "dry plating".
According to this approach, however, it is difficult to make the condition for reducing the void compatible with the condition for forming the Schottky barrier and to establish substantial repeatability. To make the two conditions compatible with each other, a special sputtering apparatus must be used.
Another solution to the problem has been to remove the flange-like portion 7 by photoetching techniques. In practice, however, it is extremely difficult to remove only the flange-like portion exactly because the width of the flange-like portion is as small as several microns and can hardly be compensated for by the precision of the existing masking technique.